The present invention relates to a heavy duty pneumatic tire having an improved belt structure.
The heavy duty pneumatic tires for heavy vehicles such as trucks, buses and the like are usually provided in the tread portion with a stiff belt, which belt includes a plurality of steel cord breaker plies. The breaker plies are made of conventional steel cords which are well known in the art such as: Regular cord (a cord in which the direction of lay in the strands is opposite to the direction of lay in closing the cord); Lang's lay cord (a cord in which the direction of lay in the strands is the same as the direction of lay in closing the cord); and Compact cord (CC) (a wrapped cord in which the filaments mainly have linear contact with each other).
Recently, a steel cord in which some (a small number) of the filaments are replaced by waving filament(s) has been proposed, for example, in U.S. Pat. No.5,351,470. This replacement of unwaving filaments with waving filaments aims to improve the penetration of rubber into the cord. Such steel cord is certainly improved in the penetration of rubber. However, due to the difference between the length of the unwaving filaments and the length of the waving filaments, a tension imbalance occurs and the cord is liable to be broken under the extreme conditions that a heavy duty pneumatic tires is often exposed.